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You should remove the gauge from your FPR under the hood or the liquid in it will evaporate. It should have came with a small plug for that port as well.
Have you seen this personally? I have ran many many cars with the liquid filled fuel gauges for years in the engine bay with zero issues.
its not really sealed, it just has a plastic plug. i have an oil pressure gauge under the hood and havent seen the fluid "evaporate". i might have leaked around the seal.
but they always have a bubble in there anyhow. maybe thats why you thought it evaporated